The following background information describes a single cylinder typically in a multiple cylinder engine, it being understood that other cylinders comprising the engine are identical and that the description applies equally well to them.
Internal combustion engines, as illustrated at 10 in FIG. 1, having hemispherical combustion chambers such as 12 atop each cylinder 13 have evolved in the quest for improved performance demanded in the racing environment. Hemi-engines, as they are more commonly known, find particular application in drag racing, where the engine is required to run at 8,600 rpm or better and produce tremendous power for a short period of time to propel the dragster at speeds in excess of 150 mph over the standing quarter mile.
Hemi-engines are preferred for racing because they are able to produce significantly more horsepower than a conventional engine for a given engine displacement. One reason for this is the placement of the intake valve 14 and exhaust valve 16 in line with the intake port 18 and exhaust port 20 of cylinder 13 which provides for a more efficient cross flow of the combustion gases through the engine.
While the hemi-engine does produce more horsepower, the cross-flow valve arrangement in combination with the hemispherical combustion chamber 12 requires that the valve stems 22 and 24 for the cylinder extend angularly from the head 26, as illustrated in FIG. 1. Rocker arms 28 and 30 associated with the cylinder 13 must be arranged to accommodate this valve stem geometry, as well as the geometry of the push rods 32 and 34 (including their lifters, not shown) which are driven from a common cam shaft (not shown), and this requirement poses difficulties in racing engine design. The difficulty is particularly acute when an existing engine, for example, the well known Chrysler 426 engine, is converted to a hemi-engine for racing use since many of the parameters affecting the geometry of the push rods and valve stems, such as the location of the cam shaft and the angle of the lifter bores (which permit the push rods to extend through the engine block to the rocker arms) are fixed and cannot be easily changed to provide a more advantageous geometrical relationship between the rocker arms and the valve stems and push rods. The traditional solution has been to arrange the rocker arms on a pair of parallel, spaced apart shafts 36, 38 arranged atop the hemi-engine head forming a stock rocker arm assembly 11. As shown in FIG. 1, the rocker arms 28 for the intake valves 14 are arranged on the common shaft 36. The rocker arms 30 for the exhaust valves 16 are arranged on a common shaft 38. Shafts 36 and 38 are supported by a plurality of individual bearings such as 39 which are bolted at points along the length of the head 26.
This common shaft arrangement for the stock rocker arm assembly 11 is disadvantageous because it is difficult to arrange the rocker arms 28 and 30, so that the point of contact between them and their respective valve stems 22 and 24 is maintained substantially coincident with the centerline of each valve stem during the rotation of the rocker arm. This is also true for the push rods 32 and 34. The contact point between the rocker arm and the push rod is not maintained coincident with the center line of the push rod.
The undesired relationship between the contact point and the valve stem centerline is illustrated in FIG. 1A, where the contact point 40 is seen to be displaced from the centerline 42 of the valve stem 24. A similar relationship occurs between the contact point 44 and the centerline 46 of the push rod 34.
When the contact point between the rocker arm and the valve stem is displaced from the centerline of the valve stem, the rocker arm does not push true along the line of motion of the valve. Significant side forces are created which impose undesired bending stresses on the valve stem and valve lifters and also cause increased friction between the valve stem and valve guides 48. Similarly, when the contact point between the push rod and the rocker arm is not substantially coincident with the centerline of the push rod, the push rod is subjected to significant side forces which increase the stresses on the push rods and the tappets.
When significant side loads are present in the elongated, reciprocating components, such as the valve stems and push rods, these components must be more robustly constructed to withstand the increased stresses, which means the components are heavier than they need be if the side loads were not present. Heavier components cannot accelerate as rapidly, thus, decreasing the speed at which they can reciprocate and inhibiting the engine from achieving its maximum rpm in the shortest possible time as well as limiting the maximum rpm. The side loads also cause additional friction between the relatively moving parts such as the valve stems and valve guides 48. This friction limits the speed at which the parts can move, thereby limiting the maximum engine speed. The friction also results in increased wear, necessitating the more frequent replacement of worn parts. Furthermore, at the high speeds which the various components move when the engine is turning maximum rpm, there is a much greater chance that valve stems or push rods could become bent by the side loads. This could lead to catastrophic failure of the engine.
The unfavorable geometry between the rocker arms, valve stems and the push rods in the stock rocker arm assembly 11 will also prevent the maximum rocker arm motion from being developed to enable the valves to operate optimally and obtain maximum engine performance. Stock rocker arms as used in the prior art assembly tend to run off of the valve stem and contact the retainer plate 41 (see FIG. 1A) long before optimal valve operation is reached. If the stock rocker arms are set up so that the rocker arm will not run off of the valve stem, the push rod angles are so severe that the push rods tend to contact the engine block. Furthermore, the unfavorable geometry will prevent taking full advantage of the rocker arm ratio, as the full effect of the ratio in moving the valves through a greater distance than the push rods move will be lessened in proportion to the angles between the various components.
Another disadvantage which results from the poor valve geometry associated with the stock rocker arm assembly is that the cam parameters, such as the duration of lift, lift rate and total lift of the valves, cannot be optimized to maximize the power output of the engine. The cam parameters are limited by the geometry of the push rods valve stems due to the side loads which are developed because the rocker arms do not push true along the centerline of the push rods and valve stems. When aggressive cam ramps which move the push rods at a relatively high lift rate are used with the stock rocker arm assembly according to the prior art, the side loads tend to severely over stress the parts and the parts fail, often with catastrophic consequences to the engine.
Yet another disadvantage of the common shaft arrangement for mounting the rocker arms is that to change a valve spring 49 for a particular valve, all of the rocker arms on the common shaft must be removed whether or not the springs related to their valves need replacement or not. Individual valve springs must be changed quite often on racing engines and it becomes a very tedious and time consuming task if all of the rocker arms on one shaft must be removed and readjusted to replace one spring. The time factor can become important if there is not enough time between heats to change a defective spring because all of the rocker arms must be removed, reinstalled and readjusted.
There is clearly room for improvement in the mounting of rocker arms on hemi-engines which allows rocker arms to be mounted individually on the head. By mounting the arms individually, more favorable geometry can be established to position and maintain the point of contact of the rocker arm in substantial coincidence with the centerline of the valve stem or the push rod. This will permit the arm to push true along the line of motion of the components and not introduce significant side loads in the components. Better geometry will also allow cam parameters to be established which will maximize engine performance. Finally, individually mounted rocker arms will allow components associated with a particular cylinder, such as the valves, valve guides or valve springs, to be changed relatively quickly without disturbing the rocker arms associated with the other cylinders.